Date of Degree

2-2020

Document Type

Dissertation

Degree Name

Ph.D.

Program

Psychology

Advisor

Jon C. Horvitz

Committee Members

Andrew Delamater

Jeff Beeler

Eleanor Simpson

Subject Categories

Behavioral Neurobiology | Biological Psychology | Experimental Analysis of Behavior | Systems Neuroscience

Keywords

Nucleus accumbens, NMDA receptors, reward, learning, plasticity, in vivo electrophysiology

Abstract

Animals learn associations between environmental cues and the natural rewards they predict (e.g., food, water, sex). As a result, reward-predictive cues come to trigger vigorous reward-seeking responses. Many neurons in the nucleus accumbens (NAc) become excited upon presentation of an already-learned reward-predictive cue. These NAc responses encode the motivational value of the cue and are necessary for the expression of the subsequent approach behavior. However, the precise temporal relationship between the emergence of cue-evoked excitations in the NAc and the acquisition of cued approach behavior remains unknown. In Experiment 1, NAc activity was recorded as rats learned to approach a reward receptacle upon presentation of a cue. The results from this experiment indicate that cue-evoked excitations begin to grow a few trials before cued approach behavior is detected and they continue to escalate as the learned response becomes more vigorous.

NAc neurons undergo N-methyl-D-Aspartate receptor (NMDAR)-mediated plasticity. However, whether NMDAR-dependent plasticity in this structure is necessary for learning to seek and secure rewards in the presence of reward-predictive stimuli remains unclear. In Experiments 2 and 3, intra-accumbens infusions of NMDAR antagonists at different points of training reveal the dynamic involvement of these receptors, untangling their specific contribution to the acquisition and expression of cued approach behavior. To understand the neural mechanisms by which NAc NMDARs participate in appetitive learning, in Experiment 4, local infusions of an antagonist were combined with electrophysiological recordings from NAc neurons during training. The results from this experiment reveal that the potentiation of training-induced cue-evoked signals in the NAc depends on NMDAR-dependent plasticity within this structure. Taken together, the results from this work link NAc plasticity, changes in NAc activity and the emergence of conditioned behavior, revealing a neural mechanism that allows animals to predict, approach and procure what is beneficial to them.

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